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Development of high-performance, hyperfluorescence OLEDs for use in display applications and solid state lighting

Periodic Reporting for period 2 - HyperOLED (Development of high-performance, hyperfluorescence OLEDs for use in display applications and solid state lighting)

Reporting period: 2018-07-01 to 2019-12-31

Organic light emitting diodes (OLEDs) have entered the mass market some years ago and can be found in various display applications ranging from microdisplays for smartglasses and electronic viewfinders, smartphones, up to TV screens. Performance of OLED based displays has improved steadily over the last years and is now on an excellent level. However, one issue not yet solved and strongly requested are highly efficient blue pixels.

The HyperOLED project targets this demand by developing blue emitting materials and matching OLED stacks which combine thermally activated delayed fluorescence (TADF) and fluorescent emitters adapted to this so-called hyperfluorescent (HF) device architecture. High performance is achieved by a novel molecular design (“shielding”) for the fluorescent emitters aimed at avoiding energy loss-mechanisms by Dexter transfer and maximising desired Förster energy transfer.

The consortium partners made significant scientific advancements in the fields of TADF, OLED devices, material synthesis, the investigation of energy transfer processes and the optical modelling of emission layers. Apart from new chemical approaches to synthesise and tune TADF materials and shielded fluorescent emitters, novel methods were developed to investigate the underlying physical mechanisms, revealing many formerly unknown effects. We could show that losses due to Dexter transfer cannot be investigated by the conventionally employed transient photoluminescence (PL) techniques and that routinely used emitter orientation measurements should be revisited to include energy transfer effects.

From the physical models of energy transfer developed during the project, we found that our shielding concept works best when a phosphorescent sensitizer is used instead of a TADF material. Using this approach, we made blue OLEDs with CIE x/y colour coordinates of 0.11/0.21 an external quantum efficiency of 17% and lifetime of 260 h (drop to 70% of initial luminance, driven with 5 mA/cm²). Based on the same material system, we have developed a single-unit white stack which shows CIE x/y of 0.40/0.42 an EQE of 21%, 43 lm/W (at 1000 cd/m², no outcoupling enhancement) and a lifetime of 860 h. We integrated a white stack in a microdisplay, but due to technical difficulties which we could identify, the expected performance was not obtained. Nevertheless, we have developed and validated the process workflow which will serve as a basis for further collaboration between the project partners in this field.
Hundreds of photo-physical and optical measurements, data from over 2000 devices using almost 50 different materials (most of them synthesised in the project) in various combinations, open scientific discussions and the diligent analysis of results significantly increased our knowledge in the project’s field of research. The drive to make this knowledge available to the whole scientific community is reflected in already 17 publications, and the submission of articles is still ongoing.

Apart from developing and applying a highly sensitive method to determine optical anisotropy, the basic assumptions of commonly used optical orientation measurements have been re-examined. We found that energy transfer processes, neglected up to now, play an important role in the interpretation of such experiments. This demonstrates the strong expertise of Fraunhofer IOF in optical analysis of OLEDs, which will attract further funding and contract research in the future.

We have developed a model to describe transient PL measurements which are routinely used in the field. The model supports various experimental evidence to prove that such measurements have fundamental limitations when it comes to the investigation of energy loss processes, making it necessary to rethink the way how HF is investigated. Nevertheless, our model allows accurate determination of the rISC and FRET rates, which are fundamental parameters governing the efficiency of HF devices. Furthermore, we could show that the rISC rate poses a limitation on how well energy losses can be suppressed by shielding the fluorescent emitter. These results once again prove the University of Durham’s leading position as a research institution in the field of photophysics of organic materials.

Our search for new materials and possibilities to tune performance resulted in nine patent applications. As an example of potential exploitation paths, we found a new class of materials which gives very colour pure deep blue emission. These materials have to be modified further in order to enter the highly attractive market for blue fluorescent emitter materials. On the material side, numerous methods to synthesize TADF and shielded fluorescent emitters were developed and structure-property relationships were identified. Purification protocols have been established in order to obtain high quality materials required for investigation in devices. The result from this work will help to speed up future progress in this area.

A highly efficient blue OLED material set was developed and integrated in a white OLED stack with a reduced number of layers and low driving voltage. Good performance was obtained in both blue and white devices which demonstrates the applicability of the project’s findings, but we acknowledge that performance in terms of colour point and device stability is not yet sufficient for application in consumer products. Nevertheless, the identification of obstacles and limitations is a highly valuable guidance for strategic decisions in further material development activities.

Due to the novelty of some materials, the use of our white stack on a micro display production line was not feasible. So, we established a workflow where coating of the OLED layers is done at Merck while finalising and characterising the wafers at MICROOLED. The resulting performance in the microdisplay device was significantly lower compared to the results obtained with test devices, and additional optimisation of the device and the process flow will be required for industrial application. However, the technical issues could be identified, and the project results serve as basis for further collaboration between the two industrial partners in the field of micro displays.
White stacked OLEDs are state-of-the art in TVs, full colour micro displays and lighting. Two or even three OLEDs are deposited on each other and connected by charge generation layers which results in a complex stack with sometimes more than 20 layers. This is necessary because fluorescent blue cannot be combined with red and green triplet emitters in a single stack due to losses by Dexter transfer. The efficient, single-unit white stack we developed demonstrates the potential of our approach to significantly reduce the number of layers in these devices. The project can thus contribute to simpler fabrication, saving time and resources in production, but also investment costs. An additional benefit is the reduction of the driving voltage required for low voltage CMOS back plane technology for micro displays. These points come on top of the improved efficiency of blue emission.

In terms of understanding, the project provides numerous novel insights and methods in the field of HF OLEDs which receive very high attention from academia and industry. Our findings will contribute to a better understanding of the underlying physical mechanisms and thus lead to an increased development speed.
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